December 21 is the shortest day of the year and the longest night of the year in the Northern Hemisphere. The winter solstice usually occurs on either December 21 or December 22.

During the winter solstice in the Northern Hemisphere the sun is the farthest south. The winter solstice occurs on June 21 or 22 in the Southern Hemisphere when the sun is the farthest north.

The solstice happens twice in a year, during summer and winter. On a solstice the sun is at its greatest distance from the celestial equator. During the summer solstice days are the longest and nights shortest and the opposite of this happens on the winter solstice.

Google is commemorating the arrival of the winter in the Northern Hemisphere with an animated winter solstice doodle showing a pair of hands weaving a long woollen hand glove that loops to form the Google letters. Google has used the same animated doodle on June 21 this year to mark the first day of winter in the Southern Hemisphere.